In inbred DBA/2 mice, the antitumor activities of separate and combined preparations of mouse immune interferon and mouse virus-induced interferon on the development of P388 tumors were studied. Immune interferon alone (25 U/day) did not affect tumor development. Virus-induced interferon alone (25,000 U/day) delayed tumor development and increased survival time. The mouse immune interferon preparations significantly enhanced or potentiated the antitumor effects of mouse virus-induced interferon when the interferons were used in combined therapy.